For reasons related essentially to development and manufacturing costs, the present trend is towards the development of CMOS technology wireless receivers, preferably as small as possible. A high level of low-frequency noise in CMOS technology transistors has led to the making of receivers based on an architecture known as low intermediate frequency architecture.
A receiver based on a low intermediate frequency architecture includes means or an input for receiving an input signal that has an input carrier frequency and is modulated by a payload signal, means or a frequency converter for changing the carrier frequency of the input signal and producing an intermediate signal that is an image of the input signal. The carrier frequency of the intermediate signal is equal to an intermediate frequency. Means or a filter circuit filters the intermediate signal, and a demodulator eliminates a frequency component, equal to the intermediate frequency, from the filtered intermediate signal to produce the payload signal.
The input analog signal received at an antenna comprises a carrier frequency signal fe, modulated by a payload signal to be detected whose frequency varies from fmin=fe−δfr to fmax=fe+δfr. In GSM telephony, for example, fe=945 MHz, and δfr=100 kHz. The input signal may also contain one or more undesired adjacent signals having a frequency above fmax or below fmin. If need be, the input signal is filtered and then pre-amplified before processing.
The input signal is then replaced by an intermediate signal having the same shape but with a carrier frequency equal to an intermediate frequency fI generally lower than the initial carrier frequency fe.
The intermediate signal is then filtered to remove especially the low-frequency noise introduced into the signal by the CMOS transistors in the receiver, and the undesired adjacent signals present in the input signal received by the antenna.
Finally, the filtered intermediate signal is demodulated by the elimination of its frequency component equal to the intermediate frequency. Thus, the payload signal is retrieved.
FIG. 1 shows an exemplary embodiment of a CMOS technology intermediate-frequency receiver. In this example, the receiver receives an analog input signal and produces an output digital payload signal. The analog/digital conversion is done simultaneously with the filtering of the intermediate signal. In this example, the change of the carrier frequency, the filtering and then the demodulation is done not directly on the pre-amplified input signal but on two signals in phase quadrature VAEQ, VAEI produced from the input signal.
The receiver of FIG. 1 thus comprises an input circuit (comprising an antenna 10, a filter 20, and a preamplifier 30) to produce an input signal VAE from the received radio signal. A first arm changes the carrier frequency of the signal VAE and produces a signal VAIQ, and filters and then demodulates the signal VAIQ. A second arm changes the carrier frequency of the signal VAE and produces a signal VAII in phase quadrature with the signal VAIQ, and filters and demodulates the signal VAII. A coupler 93 recombines the signals coming from the two arms and produces the payload signal VNU.
The means or the frequency converter for changing the frequency of the input signal comprises the following in the example of FIG. 1. A generator 40 produces two reference signals VARQ, VARI having a frequency f0 and are in phase quadrature. In the first arm, a mixer 50a mixes the input signal VAE and the first reference signal VARQ to produce a modulated intermediate signal VAIQ. In the second arm, a mixer 50b mixes the input signal VAE and the second reference signal VARI to produce a modulated intermediate signal VAII.
The signals VAIQ and VAII have a carrier frequency, called an intermediate frequency, equal to fI=|f0−fe| (absolute value of f0−fe). The generator 40 is classically a phase-locked loop type generator.
In the example of FIG. 1, the means or the filter circuit to filter the intermediate signal also converts the intermediate signal to a digital signal. The filter circuit thus comprises a bandpass analog filter 60a, 60b having a center frequency fc chosen to be equal to the intermediate frequency and having a bandwidth chosen to filter a low-frequency noise and undesired adjacent signals from the intermediate analog signal. The analog filter may be followed by an amplifier (not shown) to match the amplitude of the signals coming out of the filter with the range of operation of the following analog/digital converter.
A converter 70a, 70b converts the filtered analog signal into a corresponding digital signal. A bandpass digital filter 80a, 80b has a center frequency fa chosen such that it is equal to the intermediate frequency to eliminate the undesired adjacent signals from the intermediate digital signal.
Finally, the means or the demodulator for demodulating the filtered intermediate signal comprises the following in the example of FIG. 1. A generator 95 to produce two reference digital signals VNRQ and VNRI having a frequency f0 equal to the intermediate frequency and being in phase quadrature. In the first arm, a mixer 90a mixes the filtered intermediate signal VNIQ and the signal VNRQ, and produces a signal VNUQ. In the second arm, a mixer 90b mixes the intermediate signal VNII and the signal VNRI, and produces a signal VNUI.
After demodulation, the digital signals VNUQ and VNUI produced by each arm in phase quadrature are then recombined by the coupler 93 to produce the final digital payload signal VNU.
The filter 80a, 80b has the function of eliminating the adjacent signals that are still present in the modulated intermediate signal. These adjacent signals are undesired signals, and are present at the same time as the payload signal in the input signal is picked up on the antenna. Their presence is generally defined in the document specifying the communications standards as a function of the value of the power of the payload signal received on the antenna. Thus, for example, in the case of a GSM application, the presence of these adjacent signals is defined for a payload signal power greater than −82 dBs. Finally, the frequency of the adjacent signals is above fmax or below fmin (fmin and fmax defines the range of possible variations of the analog payload signal present in the input signal).
For example, in the case of a GSM application the range of variation of the frequency of the payload signal is contained in a 100 kHz band on either side of the carrier frequency, while the range of variation of the frequency of the adjacent signals is situated in the zone going from 100 kHz to 700 kHz on either side of the carrier frequency.
Since the mixers 50a, 50b are not in perfect phase quadrature, the adjacent signals present at the input signal are superimposed on the payload signal at output of the mixers 50a and 50b, thus creating a parasite signal in the frequency band of the payload signal. This parasite signal has a frequency whose level varies according to the value of the intermediate frequency, and on the precision of the quadrature of the mixers 50a and 50b. 
In order that the parasite signal thus generated may be weak enough to achieve the desired reception quality (defined by the communications standard considered, as also in the case of the level of the intermediate signals) it is necessary to select an intermediate frequency that is sufficiently low.
By way of indication, in the case of the GSM standard, the intermediate frequency should be below 200 kHz and, for example, an intermediate frequency of about 100 kHz is chosen.
Furthermore, in a CMOS technology receiver, the CMOS transistors introduce a low-frequency noise whose amplitude is proportional to 1/f, with f being the frequency (FIG. 2a). With the development and miniaturization of CMOS receivers, the low-frequency noise is becoming increasingly significant relative to the level of the radio signals received. This noise has become prohibitive, especially when the received radio signal is at a low or very low level (FIG. 2b). It can no longer be properly filtered out by the filters 60a, 60b of the prior-art receivers unless the manufacturing cost of these filters is greatly reduced.